Thin film transistor and display device including the same

ABSTRACT

A thin film transistor includes an active layer over a substrate, a gate electrode over the active layer, a gate line connected with the gate electrode, and a gate insulation film between the active layer and the gate electrode. The active layer includes a channel region overlapping the gate electrode, and a drain region and a source region on respective sides of the channel region. A length of a straight line connecting the drain region and the source region by a shortest distance may be greater than a width of the gate line parallel to the straight line.

CROSS-REFERENCE TO RELATED APPLICATION

Korean Patent Application No. 10-2017-0111930, filed on Sep. 1, 2017,and entitled, “Thin Film Transistor and Display Device Including theSame,” is incorporated by reference herein in its entirety.

BACKGROUND 1. Field

One or more embodiments described herein relate to a thin filmtransistor and a display device including a thin film transistor.

2. Description of the Related Art

A variety of flat panel displays have been developed. These displayshave thin film transistors, capacitors, and other elements. Each of thethin film transistors may include a channel region in an active layer, asource region, a drain region, and a gate electrode electricallyinsulated from the active layer by a gate insulation layer.

The active layer may include amorphous silicon or poly-silicon. When theactive layer includes amorphous silicon, charge mobility is low. Thus,it may be difficult to implement a driving circuit that operates at ahigh speed. When the active layer includes poly-silicon, charge mobilitymay be improved but the threshold voltage (Vth) of the thin filmtransistor may vary. Thus, a separate compensation circuit may be addedin an attempt to offset the threshold variation.

SUMMARY

In accordance with one or more embodiments, a thin film transistorincludes an active layer over a substrate; a gate electrode over theactive layer; a gate line connected with the gate electrode; and a gateinsulation film between the active layer and the gate electrode, whereinthe active layer includes a channel region overlapping the gateelectrode and a drain region and a source region on respective sides ofthe channel region, and wherein a length of a straight line connectingthe drain region and the source region by a shortest distance is greaterthan a width of the gate line parallel to the straight line.

A length of the gate electrode parallel to the straight line may begreater than a width of the gate line. The gate electrode may include aplurality of trenches toward an inside of the gate electrode from anouter side surface of the gate electrode. The plurality of trenches maypenetrate the gate electrode in a thickness direction of the gateelectrode.

The channel region may include at least one bent portion, and a lengthof the gate electrode, measured from the drain region to the sourceregion, may be greater than a length of the straight line. The gateelectrode may include a plurality of trenches toward an inside of thegate electrode from an outer side surface of the gate electrode.

The thin film transistor may include a first insulation film coveringthe gate electrode, the gate line, the source region, and the drainregion; a source electrode on the first insulation film and electricallyconnected with the source region; and a drain electrode on the firstinsulation film and electrically connected with the drain region. Theactive layer may include an oxide semiconductor. The gate electrode andthe gate line may be integrated with each other, and a shortest distancebetween any one point of the channel region and an outer side surface ofthe gate electrode may be 7 μm or less.

In accordance with one or more other embodiments, a display deviceincludes a substrate; a thin film transistor on the substrate; and adisplay element on the substrate and electrically connected with thethin film transistor. The thin film transistor includes: an active layerincluding a drain region and a source region on respective sides of achannel region; a gate electrode over the active layer and overlappingthe channel region; a gate line through which an electrical signal is tobe applied to the gate electrode; and a gate insulation film between theactive layer and the gate electrode, wherein a length of a straight lineconnecting one end of the gate insulation film adjacent to the drainregion and another end of the gate insulation film adjacent to thesource region by a shortest distance is greater than a width of the gateline parallel to the straight line.

A length of the gate insulation film parallel to the straight line maybe greater than a width of the gate line. The gate electrode may includea plurality of trenches toward the inside of the gate electrode from anouter side surface of the gate electrode. The plurality of trenches maypenetrate the gate electrode in a thickness direction of the gateelectrode. The gate insulation film may include at least one bentportion, and a length of the gate insulation film, measured from thedrain region to the source region, may be greater than a length of thestraight line. The gate electrode may include a plurality of trenchestoward an inside of the gate electrode from an outer side surface of thegate electrode.

The thin film transistor may include a first insulation film coveringthe gate electrode, the gate line, the source region, and the drainregion; a source electrode on the first insulation film and electricallyconnected with the source region; and a drain electrode on the firstinsulation film and electrically connected with the drain region,wherein the source electrode or the drain electrode is electricallyconnected with the display element. The active layer may include anoxide semiconductor.

The gate electrode and the gate line may be integrated with each other,and a shortest distance between any one point of the channel region andan outer side surface of the gate electrode may be 7 μm or less. Thedisplay device may include a buffer layer between the substrate and thinfilm transistor; and a conductive layer between the substrate and bufferlayer, wherein the conductive layer overlaps the thin film transistor.

The thin film transistor may include a first insulation film coveringthe gate electrode, the gate line, the source region, and the drainregion; a source electrode on the first insulation film and electricallyconnected with the source region; and a drain electrode on the firstinsulation film and electrically connected with the drain region,wherein the source electrode is electrically connected with theconductive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Features will become apparent to those of skill in the art by describingin detail exemplary embodiments with reference to the attached drawingsin which:

FIG. 1 illustrates an embodiment of a thin film transistor;

FIG. 2 illustrates examples of the characteristics of thin filmtransistors having different channel length;

FIG. 3 illustrates examples of electric field measurements for thin filmtransistors having different channel length;

FIGS. 4A to 4C illustrate examples of changes in threshold voltage ofthin film transistors;

FIG. 5 illustrates example of a thin film transistor for explaining achange in threshold voltage;

FIG. 6 illustrates an embodiment of a thin film transistor;

FIG. 7 illustrates an embodiment of a thin film transistor;

FIG. 8 illustrates an embodiment of a thin film transistor;

FIG. 9 illustrates an embodiment of a thin film transistor;

FIG. 10 illustrates an embodiment of a display device;

FIG. 11 illustrates an embodiment of a pixel;

FIG. 12 illustrates a view taken along section line I-I′ in FIG. 10;

FIG. 13 illustrates an example of a thin film transistor;

FIG. 14 illustrates a view taken along section line I-I′ of FIG. 10according to another embodiment; and

FIG. 15 illustrates another example of the characteristics of a thinfilm transistor.

DETAILED DESCRIPTION

Example embodiments are described with reference to the drawings;however, they may be embodied in different forms and should not beconstrued as limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will convey exemplary implementations to those skilled inthe art. The embodiments (or portions thereof) may be combined to formadditional embodiments

In the drawings, the dimensions of layers and regions may be exaggeratedfor clarity of illustration. It will also be understood that when alayer or element is referred to as being “on” another layer orsubstrate, it can be directly on the other layer or substrate, orintervening layers may also be present. Further, it will be understoodthat when a layer is referred to as being “under” another layer, it canbe directly under, and one or more intervening layers may also bepresent. In addition, it will also be understood that when a layer isreferred to as being “between” two layers, it can be the only layerbetween the two layers, or one or more intervening layers may also bepresent. Like reference numerals refer to like elements throughout.

When an element is referred to as being “connected” or “coupled” toanother element, it can be directly connected or coupled to the anotherelement or be indirectly connected or coupled to the another elementwith one or more intervening elements interposed therebetween. Inaddition, when an element is referred to as “including” a component,this indicates that the element may further include another componentinstead of excluding another component unless there is differentdisclosure.

FIG. 1 illustrates a sectional view of an embodiment of a thin filmtransistor TFT which may include an active layer A over a substrate 100,a gate electrode G over the active layer A, and a gate insulation film103 between the active layer A and the gate electrode G. The substrate100 may include a transparent glass material (e.g., SiO₂), ceramic,plastic, stainless steel, or another material.

A buffer layer 101 may be further provided on the substrate 100 in orderto impart smoothness to the substrate 100 and prevent the penetration ofimpurities. The buffer layer 101 may include an inorganic material suchas silicon nitride and/or silicon oxide. The buffer layer 101 mayinclude a single layer or a plurality of layers.

The active layer A may include an oxide semiconductor. The active layerA may include, for example, an oxide of indium (In), gallium (Ga), tin(Sn), zirconium (Zr), vanadium (V), hafnium (Hf) cadmium (Cd), germanium(Ge) chromium (Cr), titanium (Ti), and zinc (Zn). In one embodiment, theactive layer A may be an ITZO (InSnZnO) semiconductor layer or an IGZO(InGaZnO) semiconductor layer.

The gate insulation film 103 may be on the active layer A. The gateelectrode G may be at a position overlapping the active layer A, withthe gate insulation film 103 therebetween. The gate insulation film 103insulates the active layer A from the gate electrode G and may includean organic material or an inorganic material such as SiNx or SiO₂.

The gate electrode G may include a single layer or a plurality of layersincluding, for example, at least one metal of aluminum (Al), platinum(Pt), palladium (Pd), silver (Ag), magnesium (Mg), gold (Au), nickel(Ni), neodymium (Nd), iridium (Ir), chromium (Cr), calcium (Ca),molybdenum (Mo), titanium (Ti), tungsten (W), and copper (Cu). The gateelectrode G may be connected with a gate line through which anelectrical signal is applied to the gate electrode G.

The active layer A may include a channel region C overlapping the gateelectrode G. A source region S and a drain region D may be on respectivesides of the channel region G. The source region S and the drain regionD may have higher electric conductivity than the channel region C. Inone embodiment, the source region S and drain region D may haveelectrical conductivity greater than the channel region C by conductionusing plasma treatment or by impurity doping.

The channel region C may have substantially the same shape as the gateelectrode G. For example, the active layer A may be doped with animpurity, while the gate insulation film 103 is formed on the activelayer A using the gate electrode G as a self-alignment mask. The channelregion C may be formed at a position overlapping the gate electrode G.Each of the source region S and the drain region D may be doped with animpurity and formed on respective lateral sides of the channel region C.Therefore, the channel region C may have a predetermined length L1depending on the width of the gate electrode G, and characteristics ofthe thin film transistor TFT may vary depending on the length L1 of thechannel region C.

Since the gate insulation film 103 is patterned using the gate electrodeG as a mask after doping the active layer A with an impurity, the gateinsulation film 103 may also have substantially the same shape as thegate electrode G.

The thin film transistor TFT may further include a source electrode SE,and a drain electrode DE. A first insulation film 107 may cover the gateelectrode G, the source region S, and the drain region D. The sourceelectrode SE and the drain electrode DE are on the first insulation film107. The first insulation film 107 may also cover the gate line and thegate electrode G.

The first insulation film 107 may be formed, for example, with at leastone organic insulation material, e.g., polyimide, polyamide, acrylicresin, benzocyclobutene, and phenol resin. The first insulation film 107may include, for example an inorganic insulation such as SiO₂, SiNx,Al₂O₃, CuOx, Tb₄O₇, Y₂O₃, Nb₂O₅, and Pr₂O₃, and may have a multi-layerstructure in which organic insulation materials and inorganic insulationmaterials alternate.

The source electrode SE is electrically connected to the source region Sthrough the first insulation film 107. The drain electrode DE iselectrically connected to the drain region D through the firstinsulation film 107. Each of the source electrode SE and the drainelectrode DE may include a single layer or a plurality of layersincluding, for example, at least one metal of aluminum (Al), platinum(Pt), palladium (Pd), silver (Ag), magnesium (Mg), gold (Au), nickel(Ni), neodymium (Nd), iridium (Ir), chromium (Cr), calcium (Ca),molybdenum (Mo), titanium (Ti), tungsten (W), and copper (Cu). Forexample, each of the source electrode SE and the drain electrode DE mayhave a three-layer laminate structure of a titanium (Ti) layer, analuminum (Al) layer, and a titanium (Ti) layer.

FIG. 2 is a graph illustrating an example of changes in characteristicsof a thin film transistor depending on channel length of the thin filmtransistor. FIG. 3 is a graph illustrating example results of electricfield measurements depending on channel length of the thin filmtransistor. Details will be described with reference to FIGS. 2 and 3together with FIG. 1.

In FIGS. 2 and 3, (1) is a case where the channel length L1 is 5 μm, (2)is a case where the channel length L1 is 9.7 μm, and (3) is a case wherethe channel length L1 is 15.7 μm. The channel length L1 may correspondto the length of the channel region C between the source region S andthe drain region D.

In FIG. 2, a transfer curve of the thin film transistor TFT is showndepending on the channel length L1. In case (1), the driving range(v1)of the gate voltage is 2.09 V. In case (3), the driving voltagerange(v3) of the gate voltage is 3.85 V. Thus, as the channel length L1increases, the driving range of a gate voltage applied to the gateelectrode G increases. It may therefore be possible to more finelycontrol the gradation of the light emitted from the display element(e.g., OLED of FIG. 12) of the display device by changing the magnitudeof the gate voltage.

FIG. 3 shows example results of measuring an electric field depending onthe channel length L1. The electric field was measured at the boundarybetween the channel region C and the drain region D. As shown in FIG. 3,as the channel length L1 increases, the change in V_(DS) per unit lengthdecreases. Thus, the electric field at the boundary between the channelregion C and the drain region D decreases. Therefore, even when V_(DS)is large, stress due to the V_(DS) may be reduced. As a result, thereliability of the thin film transistor TFT may be improved.

As such, when the length L1 of the channel region C of the thin filmtransistor TFT increases, various characteristics of the thin filmtransistor TFT may be improved. However, when the gate electrode G(which determines the length L1 of the channel region C) and the gateline (which is capable of being integrated with the gate electrode G)are formed to have the same width, then the width of the gate line mayincrease as the length L1 of the channel region C increases. As aresult, the threshold voltage Vth may vary during the repetitivesweeping of the thin film transistor TFT.

FIGS. 4A to 4C are graphs illustrating examples of changes in thresholdvoltage of the thin film transistor depending on the width of a gateline connected with the gate electrode of the thin film transistor. FIG.5 is a plan view illustrating an example of a thin film transistor forexplaining the change in threshold voltage of the thin film transistor.Details will be described with reference to FIGS. 4A to 4C and FIG. 1.

FIG. 4A shows a case where the channel length L1 is 5 μm, FIG. 4B showsa case where the channel length L1 is 9.7 μm, and FIG. 4C shows a casewhere the channel length L1 is 15.7 μm. FIGS. 4A to 4C show the resultsof sweeping the V_(DS) six times from 0.1 V to 5.1 V, respectively. Asshown in FIGS. 4A to 4C, it can be seen that as the channel length L1increases, the threshold voltage Vth of the thin film transistor TFTshifts to the right during the repetitive sweeping. The reason for thismay be that, as the channel length L1 increases (e.g., as the width ofthe gate electrode G increases), electron trapping occurs at theinterface between the channel region C and the gate insulation film 103(e.g., see FIG. 1). One possible cause of the electronic trapping may bedescribed with reference to FIG. 5.

The thin film transistor TFT is subject to a heat treatment processafter forming the first insulation film 107. During the heat treatmentprocess, hydrogen in the first insulation film 107 diffuses into thechannel region C to prevent trapping of electrons in the channel regionC. However, as shown in FIG. 5, when the width W1 of the gate electrodeG is equal to the width W2 of the gate line GL, then the width W2 of thegate line GL may increase as the width W1 of the gate electrode Gincreases. Thus, during the heat treatment process after forming thefirst insulation film 107, the distance from the intersection of thegate line GL and the active layer A to the center of the channel regionC increases. As a result, the diffusion distance of hydrogen increases,so that a region V into which hydrogen does not sufficiently diffuse maybe formed in the central region of the channel region C. Therefore, thelength of the channel region C may be increased and the hydrogen may bediffused into the entire channel region C during the heat treatmentafter forming the first insulation film 107.

FIGS. 6 to 9 are plan views illustrating respective examples of the thinfilm transistor of FIG. 1. Referring to FIG. 6, the length of a straightline connecting the drain region D and source region S of the thin filmtransistor TFT by the shortest distance may be greater than the width W2of the gate line GL. The width W2 of the gate line GL may correspond tothe width measured in a direction parallel to the straight line. Forexample, the gate electrode G may extend toward the source region S andthe drain region D in a direction parallel to the straight line. Thus,the length L1 of the channel region C having the same shape as the gateelectrode G increases. This may increase the driving range of the gatevoltage applied to the gate electrode G of the thin film transistor TFTand improve reliability of the thin film transistor TFT. Further, asdescribed above, since the gate insulation film 103 (e.g., see FIG. 1)may also have substantially the same shape as the gate electrode G, thelength of a straight line connecting the one end of the gate insulationfilm 103 (e.g., see FIG. 1) adjacent to the drain region D and the otherend of the gate insulation film 103 (e.g., see FIG. 1) adjacent to thesource region S may be greater than the width W2 of the gate line GL.

Since the gate line GL has a smaller width W2 than the gate electrode G,the distance from the intersection of the gate line GL and the activelayer A to the center of the channel region C may not increase or maydecrease, even when the length L1 of the channel region C increases.Therefore, during the heat treatment after the formation of the firstinsulation film 107 (e.g., see FIG. 1), hydrogen may be diffused intothe entire channel region C, thereby preventing the occurrence ofelectron trapping at the interface between the channel region C and thegate insulation film 103 (e.g., see FIG. 1). Therefore, the change inthe threshold voltage Vth of the thin film transistor TFT may be reducedor minimized, even during repetitive sweeping.

During the heat treatment after the formation of the first insulationfilm 107 (e.g., see FIG. 1), in order for hydrogen to be moreeffectively diffused into the entire channel region C, the shortestdistance between any one point of the channel region C and the outerside surface of the gate electrode G may be 7 μm or less.

The center of the channel region C is a point that spaced farthest awayfrom the outer side surface of the gate electrode G. If the distancefrom the intersection of the gate line GL and the channel region C tothe center of the channel region C is 7 μm or less, the entire channelregion C is spaced from the outer side surface of the gate electrode Gby a distance of 7 μm or less. Thus, hydrogen may be effectivelydiffused into the entire channel region C during the heat treatmentafter the formation of the first insulation film 107 (e.g., see FIG. 1).The aforementioned range is an example range and different ranges mayapply in one or more embodiments.

Referring to FIG. 7, a transistor TFTB is different from that of FIG. 6in that the gate electrode G includes a plurality of first trenches H1.The plurality of first trenches H1 may be formed toward the inside ofthe gate electrode G from the outer side surface of the gate electrodeG. The plurality of first trenches H1 may penetrate the gate electrode Gin the thickness direction of the gate electrode G. In one or moreembodiments, the gate electrode G includes a plurality of protrusions Pprotruding in a direction perpendicular to the length L1 of the channelregion C.

Therefore, even when the length L1 of the channel region C increases,hydrogen may be effectively diffused into the entire channel region Cduring the heat treatment after the formation of the first insulationfilm 107 (e.g., see FIG. 1). Further, since the plurality of protrusionsP among the plurality of first trenches H1 may extend to the outside ofthe channel region C in a direction perpendicular to the direction ofthe length L1 of the channel region C, it may be possible to prevent achange in characteristics of the thin film transistor TFTB fromoccurring, even when an error occurs in alignment between the gateelectrode G and the channel region C during formation of the gateelectrode G.

Referring to FIG. 8, the channel region C of the thin film transistorTFTC may include at least one bent portion. Also, the length L1 of thechannel region C having the same shape as the gate electrode G may begreater than the length of a straight line connecting the drain region Dand the source region S by the shortest distance. Therefore, the lengthL1 of the channel region C may be increased or maximized within alimited area. FIG. 8 illustrates a shape in which the channel region Cis bent three times. The channel region C may have a different shape(e.g., “S”, “M”, “W,” etc.) in other embodiments.

Referring to FIG. 9, a transistor TFTD is different from that of FIG. 8in that the gate electrode G includes a plurality of second trenches H2.The plurality of second trenches H2 may be formed toward the inside ofthe gate electrode G from the outer side surface of the gate electrodeG, and may penetrate the gate electrode G in the thickness direction ofthe gate electrode G. The gate electrode G may include a plurality ofprotrusions P protruding outward at positions among the plurality ofsecond trenches H2. A plurality of second grooves H1 may expose a partof the channel region C. The protrusions P among the plurality of secondtrenches H2 may extend to the outside of the channel region C.Therefore, it may be possible to prevent a change in characteristics ofthe thin film transistor TFTD from occurring, even if an error occurs inthe alignment between the gate electrode G and the channel region Cduring formation of the gate electrode G.

FIG. 10 illustrates an embodiment of a display device, and FIG. 11illustrates an equivalent circuit of a pixel which, for example, may beincluded in the display device of FIG. 10.

Referring to FIG. 10, an active area AA for displaying an image and adead area DA adjacent to the active area AA are on the substrate 100 ofthe organic light-emitting display device 1. The active area AA includespixel areas PA. A pixel for emitting predetermined light is formed foreach of the pixel areas PA. An image is generated based on light emittedfrom the plurality of pixels in the active area AA.

The dead area DA may surround the active area AA and may include adriving unit for transmitting a predetermined signal to the pixels inthe active area AA.

A protective substrate may be on the substrate 100 to protect the activearea AA from external foreign matter. A sealing member may be betweenthe substrate 100 and the protective substrate and may surround theactive area AA. In one embodiment, a thin sealing film may be on theactive area AA to protect the active area AA from external foreignmatter.

Referring to 11, each pixel may include a display element to emit lightwith predetermined luminance by a switching thin film transistor TFT1, adriving thin film transistor TFT2, a storage capacitor Cst, and adriving current (Ioled). The display element may be an organiclight-emitting diode OLED.

The switching thin film transistor TFT1 is connected to a scan line SLnand a data line DLm, and transmits a data signal input to the data lineDLm based on a scan signal input to the scan line SLn to the drivingthin film transistor TFT2.

The storage capacitor Cst is connected to the switching thin filmtransistor TFT1 and a first voltage line PL, and stores a voltagecorresponding to a difference between a voltage from the switching thinfilm transistor TFT1 and a first power voltage ELVDD supplied to thefirst voltage line PL.

The driving thin film transistor TFT2 may be connected to the firstvoltage line PL and the storage capacitor Cst, and may control a drivingcurrent flowing from the first voltage line PL to the organiclight-emitting diode OLED based on a voltage value stored in the storagecapacitor Cst. The organic light-emitting diode OLED may emit lighthaving predetermined luminance by the driving current. The pixel mayhave a different configuration (e.g., a different number of transistorsand/or capacitors) in another embodiment.

The driving thin film transistor TFT2 may have a configuration whichcorresponds to one of the embodiments illustrated in FIGS. 6 to 9. Thus,the driving range of the gate voltage applied to the gate electrode ofthe driving thin film transistor TFT2 may be widened.

FIGS. 12 and 13 illustrate one or more embodiments of the pixel area PA.In FIG. 12, for the convenience of explanation, in the pixel circuit ofFIG. 11, the switching thin film transistor TFT1 is omitted, and thedriving thin film transistor TFT2 is referred to as a thin filmtransistor TFT2.

The substrate 100 may include various materials. For example, thesubstrate 100 may include a transparent glass material (e.g., SiO₂) or atransparent plastic material. The plastic material may be, for example,polyether sulphone (PES), polyacrylate (PAR), polyether imide (PEI),polyethylene naphthalate (PEN), polyethyelene terephthalate (PET),polyphenylene sulfide (PPS), polyallylate, polyimide, polycarbonate(PC), cellulose triacetate (TAC), or cellulose acetate propionate (CAP).

In the case of a back emission-type display device in which an image isrealized in the direction of the substrate 100, the substrate 100includes a transparent material. However, in the case of a frontemission-type display device in which an image is realized in adirection opposite to the substrate 100, the substrate 100 may or maynot include a transparent material. In this case, in one embodiment, thesubstrate 100 may include a metal, e.g., iron, chromium, manganese,nickel, titanium, molybdenum, stainless steel (SUS), Invar alloy,Inconel alloy, or Kovar alloy.

A buffer layer 101 may be on the substrate 100, may provide a flatsurface on the substrate 100, and may block foreign matter or moisturepenetrating through the substrate 100. For example, the buffer layer 101may include an inorganic material (e.g., silicon oxide, silicon nitride,silicon oxynitride, aluminum oxide, aluminum nitride, titanium oxide, ortitanium nitride) or an organic material such as polyimide, polyester,or acrylate. The buffer layer 101 may be a laminate of a plurality oflayers including the aforementioned materials.

A display element may be electrically connected with a thin filmtransistor TFT2 over the substrate 100. The thin film transistor TFT2 myinclude an active layer A, a gate electrode G over the active layer A,and a gate insulation film 103 between the active layer A and the gateelectrode G. The active layer A may include an oxide semiconductor,e.g., indium (In), gallium (Ga), tin (Sn), zirconium (Zr), vanadium (V),hafnium (Hf) cadmium (Cd), germanium (Ge) chromium (Cr), titanium (Ti),and zinc (Zn). For example, the active layer A may be an IGZO (InGaZnO)semiconductor layer.

The active layer A may include a channel region C overlapping the gateelectrode G, and a source region S and a drain region D on respectivesides of the channel region G.

The gate insulation film 103 may include an organic material or aninorganic material such as SiNx or SiO₂.

The gate electrode G is on the gate insulation film 103 and may beconnected with a gate line GE for applying on/off signals to the thinfilm transistor TFT2. The gate electrode G may include a single layer ora plurality of layers including, for example, at least one metal ofaluminum (Al), platinum (Pt), palladium (Pd), silver (Ag), magnesium(Mg), gold (Au), nickel (Ni), neodymium (Nd), iridium (Ir), chromium(Cr), calcium (Ca), molybdenum (Mo), titanium (Ti), tungsten (W), andcopper (Cu).

The width W1 of the gate electrode G may be greater than the width W2 ofthe gate line GE. Thus, the trapping of electrons in the channel regionC may be prevented even when the length of the channel region Cincreases. Thus, the driving range of a gate voltage increases. Thus,the gradation of the light emitted from a display element 200 may bemore finely controlled by changing the magnitude of the gate voltage.Also, the resolution and display quality of the flat panel displaydevice 1 (e.g., see FIG. 10) may be improved. For this purpose, the gateelectrode G may have the shape described in FIGS. 6 to 9, and theshortest distance between any one point of the channel region C and theouter side surface of the gate electrode G may be 7 μm or less.

First and second insulation films 107 and 108 are formed on the gateelectrode G. In this embodiment, the two interlayer insulation films(e.g., the first and second insulation films 107 and 108) may be on thegate electrode G. In one embodiment, the gate electrode G may be coveredwith a single interlayer insulation film.

A source electrode SE and a drain electrode DE may be electricallyconnected to the source region S and the drain region D, respectively,and may be on the second insulation layer 108. Each of the sourceelectrode SE and the drain electrode DE may include a single layer or aplurality of layers including, for example, at least one metal ofaluminum (Al), platinum (Pt), palladium (Pd), silver (Ag), magnesium(Mg), gold (Au), nickel (Ni), neodymium (Nd), iridium (Ir), chromium(Cr), calcium (Ca), molybdenum (Mo), titanium (Ti), tungsten (W), andcopper (Cu). For example, each of the source electrode SE and the drainelectrode DE may have a three-layer laminate structure, for example, oftitanium (Ti), aluminum (Al), and titanium (Ti).

A planarization layer 109 may be over the thin film transistor TFT2. Theplanarization layer 109 may eliminate a step caused by the thin filmtransistor TFT2 and may planarize the upper surface of the thin filmtransistor TFT2, thereby preventing the display element 200 from beingdefective due to the lower unevenness.

The planarization layer 109 may be a single layer or a multi-layerincluding an organic material. Examples of the organic material mayinclude, for example, a general purpose polymer, e.g., polymethylmethacrylate (PMMA) and polystyrene (PS), polymer derivatives having aphenolic group, acrylic polymers, imide-based polymers, aryl etherpolymers, amide-based polymers, fluorine-based polymers, p-xylene-basedpolymers, vinyl alcohol-based polymers, or blends thereof. Further, theplanarization layer 109 may be a composite laminate of an inorganicinsulation film and an organic insulation film.

The display element 200 is on the planarization layer 109. In oneembodiment, the display element 200 may be an organic light-emittingelement including a first electrode 210, a second electrode 230 facingthe first electrode 210, and an intermediate layer 220 between the firstelectrode 210 and the second electrode 230.

The first electrode 210 may be on the planarization layer 109 andelectrically connected to the thin film transistor TFT2. The firstelectrode 210 may be a reflective electrode. In one embodiment, thefirst electrode 210 may include a reflective film of, for example, Ag,Mg, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr, or a compound thereof, and atransparent or translucent electrode layer on the reflective film. Thetransparent or translucent electrode layer may include at least one ofindium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO),indium oxide (In₂O₃), indium gallium oxide indium gallium oxide, andaluminum zinc oxide (AZO).

The second electrode 230 may be a transparent or translucent electrode.In one embodiment, the second electrode 230 may include a metal thinfilm having a low work function and including Li, Ca, LiF/Ca, LiF/Al,Al, Ag, Mg, and a compound thereof. An auxiliary electrode layer or abus electrode, which includes a material for forming a transparentelectrode (e.g., ITO, IZO, ZnO, or In₂O₃) may be on the metal thin film.Accordingly, the second electrode 230 may transmit the light emittedfrom the organic light-emitting layer in the intermediate layer 220. Forexample, the light emitted from the organic light-emitting layer may bedirectly reflected or reflected by the first electrode 210 including thereflective electrode and may be emitted toward second electrode 230.

In some embodiments, the display device 1 (e.g., see FIG. 10) may be afront emission-type display device and a back emission-type displaydevice in which the light emitted from the organic light-emitting layeris emitted toward the substrate 100. The first electrode 210 may be atransparent or translucent electrode. The second electrode 230 may be areflective electrode. Further, the display device 1 (e.g., see FIG. 10)of this embodiment may be a dual side emission-type display device inwhich light is emitted in both front and back directions.

A pixel-defining layer 110 is on the first electrode 210 and may includean insulation material. For example, the pixel-defining layer 110 mayinclude at least one organic insulation material, e.g., polyimide,polyamide, acrylic resin, benzocyclobutene, or phenol resin by a methodsuch as spin coating. The pixel-defining layer 110 exposes apredetermined area of the first electrode 210, and the intermediatelayer 220 including an organic light-emitting layer is in the exposedarea. The pixel-defining layer 110 defines a pixel area of the organiclight-emitting element.

The organic light-emitting layer in the intermediate layer 220 mayinclude a low molecular weight organic material or a high molecularweight organic material. The intermediate layer 220 may furtherselectively include a functional layer such as a hole transport layer(HTL), a hole injection layer (HIL), an electron transport layer (ETL),or an electron injection layer (EIL) in addition to the organiclight-emitting layer.

FIG. 14 illustrates another embodiment taken along section line I-I′ ofFIG. 10. FIG. 15 is a graph illustrating an example of the changes incharacteristics of a thin film transistor depending on the presence orabsence of a blocking layer of FIG. 14. FIG. 14 illustrates only thedifference from FIG. 12.

Referring to FIG. 14, the display device may further include aconductive layer 400 between the substrate 100 and the buffer layer 101.The conductive layer 400 overlaps the thin film transistor TFT2 andprevents light from entering the thin film transistor TFT2. Therefore, aphotocurrent is in the oxide semiconductor of the thin film transistorTFT2 due to the incidence of light, thereby preventing thecharacteristics of the thin film transistor TFT2 from deteriorating.

Further, the conductive layer 400 is electrically connected to thesource electrode SE of the thin film transistor TFT2, to further improvethe characteristics of the thin film transistor TFT2.

In FIG. 15, the dashed line indicates a state in which the conductivelayer 400 is electrically connected to the source electrode SE and thesolid line indicates a state in which the conductive layer 400 is notformed. FIG. 15 shows example results of measuring drain current whenVgs of 1.1 V, 5.1 V, and 7.1 V are applied in the upward direction fromthe bottom of FIG. 15. As described above, it can be seen that it iseasy to secure a stable saturation area when the conductive layer 400and the source electrode SE are connected to each other.

According to one or more of the aforementioned embodiments, the lengthof the channel of a thin film transistor is increased. Thus, the drivingrange of the gate voltage applied to the gate electrode of the thin filmtransistor may be widened. Further, the width of the gate electrode isgreater than the width of the gate line connected with the gateelectrode. Thus, the change in threshold voltage Vth of the thin filmtransistor may be reduced or minimized even during repetitive sweeping.

Example embodiments have been disclosed herein, and although specificterms are employed, they are used and are to be interpreted in a genericand descriptive sense only and not for purpose of limitation. In someinstances, as would be apparent to one of skill in the art as of thefiling of the present application, features, characteristics, and/orelements described in connection with a particular embodiment may beused singly or in combination with features, characteristics, and/orelements described in connection with other embodiments unless otherwiseindicated. Accordingly, various changes in form and details may be madewithout departing from the spirit and scope of the embodiments set forthin the claims.

What is claimed is:
 1. A thin film transistor, comprising: an activelayer over a substrate; a gate electrode over the active layer; a gateline electrically connected with the gate electrode; and a gateinsulation film between the active layer and the gate electrode, whereinthe active layer includes a channel region overlapping the gateelectrode and a drain region and a source region on respective sides ofthe channel region, wherein a length of a straight line connecting thedrain region and the source region by a shortest distance is greaterthan a width of the gate line parallel to the straight line, andwherein: i) the gate electrode includes a plurality of trenches towardan inside of the gate electrode from an outer side surface of the gateelectrode, or ii) the channel region includes at least one bent portion,and a length of the gate electrode, measured from the drain region tothe source region, is greater than a length of the straight line.
 2. Thethin film transistor as claimed in claim 1, wherein a length of the gateelectrode parallel to the straight line is greater than a width of thegate line.
 3. The thin film transistor as claimed in claim 1, wherein:the gate electrode includes the plurality of trenches, and the pluralityof trenches penetrate the gate electrode in a thickness direction of thegate electrode.
 4. The thin film transistor as claimed in claim 1,wherein: the channel region includes the at least one bent portion, andthe gate electrode includes the plurality of trenches toward the insideof the gate electrode from the outer side surface of the gate electrode.5. The thin film transistor as claimed in claim 1, further comprising: afirst insulation film covering the gate electrode, the gate line, thesource region, and the drain region; a source electrode on the firstinsulation film and electrically connected with the source region; and adrain electrode on the first insulation film and electrically connectedwith the drain region.
 6. The thin film transistor as claimed in claim1, wherein the active layer includes an oxide semiconductor.
 7. The thinfilm transistor as claimed in claim 1, wherein: the gate electrode andthe gate line are integrated with each other, and a shortest distancebetween any one point of the channel region and an outer side surface ofthe gate electrode is 7 μm or less.
 8. A display device, comprising: asubstrate; a thin film transistor on the substrate; and a displayelement on the substrate and electrically connected with the thin filmtransistor, wherein the thin film transistor includes: an active layerincluding a drain region and a source region on respective sides of achannel region; a gate electrode over the active layer and overlappingthe channel region; a gate line through which an electrical signal is tobe applied to the gate electrode; and a gate insulation film between theactive layer and the gate electrode, wherein a length of a straight lineconnecting one end of the gate insulation film adjacent to the drainregion and another end of the gate insulation film adjacent to thesource region by a shortest distance is greater than a width of the gateline parallel to the straight line, and wherein: i) the gate electrodeincludes a plurality of trenches toward an inside of the gate electrodefrom an outer side surface of the gate electrode, or ii) the channelregion includes at least one bent portion, and a length of the gateelectrode, measured from the drain region to the source region, isgreater than a length of the straight line.
 9. The display device asclaimed in claim 8, wherein a length of the gate insulation filmparallel to the straight line is greater than a width of the gate line.10. The display device as claimed in claim 8, wherein: the gateelectrode includes the plurality of trenches, and the plurality oftrenches penetrate the gate electrode in a thickness direction of thegate electrode.
 11. The display device as claimed in claim 8, wherein:the channel region includes the at least one bent portion, and the gateelectrode includes the plurality of trenches toward the inside of thegate electrode from the outer side surface of the gate electrode. 12.The display device as claimed in claim 8, wherein the thin filmtransistor includes: a first insulation film covering the gateelectrode, the gate line, the source region, and the drain region; asource electrode on the first insulation film and electrically connectedwith the source region; and a drain electrode on the first insulationfilm and electrically connected with the drain region, wherein thesource electrode or the drain electrode is electrically connected withthe display element.
 13. The display device as claimed in claim 8,wherein the active layer includes an oxide semiconductor.
 14. Thedisplay device as claimed in claim 8, wherein: the gate electrode andthe gate line are integrated with each other, and a shortest distancebetween any one point of the channel region and an outer side surface ofthe gate electrode is 7 μm or less.
 15. The display device as claimed inclaim 8, further comprising: a buffer layer between the substrate andthe thin film transistor; and a conductive layer between the substrateand the buffer layer, wherein the conductive layer overlaps the thinfilm transistor.
 16. The display device as claimed in claim 15, whereinthe thin film transistor includes: a first insulation film covering thegate electrode, the gate line, the source region, and the drain region;a source electrode on the first insulation film and electricallyconnected with the source region; and a drain electrode on the firstinsulation film and electrically connected with the drain region,wherein the source electrode is electrically connected with theconductive layer.
 17. A display device, comprising: a substrate; a thinfilm transistor on the substrate; and a display element on the substrateand electrically connected with the thin film transistor, wherein thethin film transistor includes: an active layer including a drain regionand a source region on respective sides of a channel region; a gateelectrode over the active layer and overlapping the channel region; agate line through which an electrical signal is to be applied to thegate electrode; a gate insulation film between the active layer and thegate electrode; a buffer layer between the substrate and the thin filmtransistor; and a conductive layer between the substrate and the bufferlayer, wherein the conductive layer overlaps the thin film transistor,wherein a length of a straight line connecting one end of the gateinsulation film adjacent to the drain region and another end of the gateinsulation film adjacent to the source region by a shortest distance isgreater than a width of the gate line parallel to the straight line, andwherein the thin film transistor includes: a first insulation filmcovering the gate electrode, the gate line, the source region, and thedrain region; a source electrode on the first insulation film andelectrically connected with the source region; and a drain electrode onthe first insulation film and electrically connected with the drainregion, wherein the source electrode is electrically connected with theconductive layer.